Sigma-delta modulators are now widely used for conversion between analog and digital signals, especially with advances in very large-scale integrated circuit technology (VLSI).
An article by P. M. Aziz, H. V. Sorensen and J. van der Spiegel in IEEE Signal Processing magazine, January 1996 gives an overview of analog-to-digital sigma-delta modulators as used in analog-to-digital converters for example. An article by P. F. Ferguson, Jr. A. Ganesan and R. W. Adams, “One Bit Higher-Order Sigma-Delta A/D Converters”, IEEE International Symposium on Circuits and Systems, pp. 890–893, 1990 gives a presentation of the general form of higher-order signal-delta modulators including filters in both feed-forward and feedback paths.
Two basic kinds of sigma-delta modulators exist: discrete-time and continuous-time. FIG. 1 of the accompanying drawings shows a typical basic discrete-time sigma-delta modulator of second order (that is to say comprising two integrator stages) and FIG. 2 shows the circuit configuration of a typical switched-capacitor integrator stage in the modulator of FIG. 1. FIG. 3 shows the transfer functions of a typical basic second order continuous-time sigma-delta modulator.
In general terms, an analog-to-digital sigma-delta modulator receives analog input signals X (that is to say whose amplitude represents data) and converts them at clock intervals to encoded digital output signals Y (that is to say pulses whose amplitude is constant and whose repetition rate represents the data). The modulator comprises a feedback path 1 for producing analog feedback signals that are a function of the digital output signals, an integrator 2 for integrating analog difference signals that are a difference function of the analog input signal and the analog feedback signals, and a quantizer 3 responsive to the signals integrated by the integrator 2 for producing the digital output signals at clock intervals defined by a clock signal CK.
An article by D. K. Su and B. A. Wooley, “A CMOS Oversampling D/A Converter with a Current-Mode Semidigital Reconstruction Fllter”, IEEE Journal of Solid-State Circuits, Vol. 28, No. 12, December 1993, pp. 1224–1233 gives a description of a digital-to-analog modulator including a finite impulse response (‘FIR’) filter, in particular a semi-digital FIR filter in the output path. The technique proposed is not applicable to an analog-to-digital modulator.
In U.S. Pat. No. 5,357,252, assigned to the assignee of the present invention, first-order FIR filtering in the feedback path of the first stage of an analog-to-digital modulator is proposed to combat the pattern noise. This method changes the noise transfer function of the modulator and its extension to higher-order filtering is not practicable.
An article by T. Okamoto, Y. Maruyama and A. Yukawa, “A Stable High-Order-Delta-Sigma Modulator with an FIR Spectrum Distributor”, IEEE Journal of Solid-State Circuits, Vol. 28, No. 7, pp. 730–735, July 1993, describes a noise-shaper circuit including an FIR spectrum distributor, used to improve the stability of higher-order modulators. The order of FIR filtering is limited to twice the modulator order. The orders of the successive FIR filters are not the same but are stepped from one up to the modulator order along the feedback path. No improvement in terms of power or distortion is apparent in this architecture. It is noted that the article describes a digital-to-analog modulator whose feedback path includes a plurality of feedback stages including finite impulse response filters of differing orders.
Concerns that arise in the design of analog-to-digital sigma-delta modulators include their sensitivity to the effects of feedback voltage step changes and clock pulse instabilities. The present invention provides novel analog-to-digital sigma-delta modulators that address these concerns, among others.